I am running Dovecot as mailserver and had the problem that I wasn’t able to remove a subfolder in my Inbox. Thunderbird just prompted the error that the action could not be performed. So I decided to remove the folder myself and this is how you do it:
1. Go to your maildir. In my case it’s /var/vmail if you have a divergent configuration have a look at your configuration file
2. Navigate to your mailserver and the account you want to do the change to
3. Find the file subscriptions
This file contains every (sub)folder you want to fetch with your mail client. So even the Trash and Sent folders are listed here. To unsubscribe from one Folder just remove the entry from the file. Then you need to restart your mailclient because it only fetches the subfolders at startup.
The benefit of this way is that you don’t lose the messages of the subfolder. You can re-subscribe to it everytime you want by just adding the name of the folder again.
Hint: make sure to check the file permissions of subscriptions afterwards. It needs to belong to user and group vmail